Recently, a diet that lowers intake of a combination of foods has been found to be effective in adults with IBS identified with fructose malabsorption. This diet has not been used in children with IBS nor has its mechanism(s) of efficacy been explored. This pilot project focuses on using a restricted fermentable substrate diet as a treatment in children with IBS, while evaluating decreased bacterial fermentation gas production and decreased gastrointestinal inflammation as mechanisms of its effect.